Recyclable hybrid organocatalysts for the Knoevenagel condensation
Knoevenagel condensation is one of the most used methods in the preparation of substituted alkenes, which permits the formation of double carbon-carbon bonds from carbonylic or heterocarbonylic compounds and an active methylene compound. Traditionally, primary or secondary amines or buffer systems containing ammonia or amines and the corresponding conjugated acid have been used as catalysts.
Imidazolium and dihidroimidazolium salts are good organocatalysts in this type of reactions. In organocatalysis, where an organic compound is the responsible of increasing the reaction rate, metallic species are omitted. Thus, this kind of catalysis is considered environmentally friendly.
In our case, organic-inorganic hybrid silica materials have been prepared from imidazolium and dihydroimidazolim salts (see scheme). Immobilization of the catalysts to a solid support offers some advantages. They can be easily separated by filtration facilitating the catalyst recovery and reuse.
The catalytic activity and recyclability of these materials in the Knoevenagel condensation of malononitrile and ethyl acetate with aromatic aldehydes under solvent-free conditions have been studied. Our heterogeneous catalysts (0.4-1.2 %) present higher activities than related homogeneous imidazolium salts. The best performances were obtained with the material derived from polycondensation of the disilylated dihydroimidazolium salt in the absence of tetraethoxysilane (TEOS). In some cases the catalysts have been reused until 5 runs without a decrease in its activity.
"Organic-inorganic hybrid silica materials containing imidazolium and dihydroimidazolium salts as recyclable organocatalysts for Knoevenagel condensations". Trilla, M.; Pleixats, R.; Wong Chi Man, M.; Bied, C. Green Chem. 2009, 11, 1815-1820.